1. Field of the Invention
The present invention relates to a pixel circuit for current-driving a light-emitting device in each pixel. The invention relates particularly to an active pixel circuit which controls the amount of current supplied to a light-emitting device such as organic EL device using insulated gate field effect transistors disposed in the pixel circuit. The invention relates more specifically to a technique of correcting variations in mobility of a drive transistor adapted to drive a light-emitting device formed in each pixel circuit.
2. Description of the Related Art
In an image display apparatus such as liquid crystal display, a number of liquid crystal pixels are arranged in a matrix form. An image is displayed on such a display device by controlling the transmitted or reflected intensity of the incident beam for each pixel according to the image information to be displayed. The same holds true for an organic EL display using organic EL devices as its pixels, except that it is a self light-emitting device. For this reason, organic EL displays offer advantages over liquid crystal displays, including higher image visibility, no necessity of backlight and higher response speed. Further, the brightness level (grayscale) of each light-emitting device can be controlled by adjusting the current flowing through the device. Organic EL displays are significantly different from voltage-controlled displays such as liquid crystal displays in that they are so-called current-controlled displays.
As with liquid crystal displays, there are two methods of driving organic EL displays, namely, simple matrix and active matrix. Despite its simplicity in structure, the former has several problems, including difficulties in providing a large-sized display with high definition. Therefore, development activities for active matrix displays are proceeding at a brisk pace. This driving method is designed to control the current flowing through the light-emitting device in each pixel circuit using active devices (generally thin film transistors or TFTs) provided in the pixel circuit. An active pixel circuit is disclosed in the following Japanese Patent Laid-Open No. Hei 8-234683 (referred to as Patent Document 1), JP-A-2002-514320, and Japanese Patent Application Laid-Open No. 2005-173434 (hereinafter referred to as Patent Document 2, and Patent Document 3, respectively).
FIG. 1 is a circuit diagram illustrating the simplest configuration of a pixel circuit in the past. As shown in the figure, the pixel circuit is disposed where a scan line, arranged in a row direction to supply a control signal, and a data line, arranged in a column direction to supply a video signal, intersect each other. The pixel circuit includes a sampling transistor T4, a capacitor C, a drive transistor T1 and a light-emitting device OLED. The light-emitting device is, for example, an organic EL device. The sampling transistor T4 conducts in response to the control signal from the scan line so as to sample the video signal from the data line. The capacitor C retains an input voltage commensurate with the video signal sampled. The drive transistor T1 supplies an output current during a given light-emitting period in accordance with the input voltage retained by the capacitor C. It is to be noted that the output current typically has dependence on a carrier mobility p in the channel region of the drive transistor T1 and a threshold voltage Vth of the same transistor T1. The light-emitting device OLED emits light at the brightness commensurate with the video signal by the output current from the drive transistor T1. It is to be noted that, in the example illustrated, one current path end (source) of the drive transistor T1 is connected to a power supply potential VDD, and the other current path end (drain) to the anode of the light-emitting device OLED. The cathode of the light-emitting device OLED is connected to a ground potential GND.
As the input voltage, retained by the capacitor C, is applied to a gate G of the drive transistor T1, the transistor T1 allows an output current to flow from its source to its drain, thus supplying the current to the light-emitting device OLED. Typically, the light-emission brightness of the light-emitting device OLED is proportional to the amount of current supplied. Further, the amount of output current supplied from the drive transistor T1 is controlled according to a gate voltage, that is to say, the input voltage written to the capacitor C. With a pixel circuit in the past, the amount of current supplied to the light-emitting device OLED is controlled by varying the input voltage applied to the gate G of the drive transistor T1 according to the input video signal.
Here, the operating characteristic of the drive transistor T1 is expressed by a formula 1 shown below.Ids=(½)μ(W/L)Cox(Vgs−Vth)2  (1)
In this transistor characteristic formula 1, Ids is a drain current flowing from the source to the drain. This current is an output current supplied to the light-emitting device OLED in the pixel circuit. Vgs is a gate voltage applied to the gate relative to the source. In the pixel circuit, Vgs is the aforementioned input voltage. Vth is a transistor threshold voltage. μ is a mobility of a semiconductor thin film making up a transistor channel. W is a channel width, L a channel length, and Cox a gate capacitance. As is clear from the transistor characteristic formula 1, if the gate voltage Vgs exceeds the threshold voltage Vth during the operation of a thin film transistor in a saturated region, the transistor turns on, causing the drain current Ids to flow. In terms of the operating principle, the same amount of the drain current Ids is supplied to the light-emitting device OLED at all times so long as the gate voltage Vgs remains constant, as shown in the transistor characteristic formula 1. Therefore, if a video signal having the same level is supplied to all pixels making up the screen, all the pixels will emit light at the same brightness. This should provide a screen uniformity.
In reality, however, thin film transistors (TFTs) which include semiconductor thin films such as polysilicon vary one from another in device characteristics. In particular, the threshold voltage Vth is not constant and instead varies from one pixel to. another. As is clear from the transistor characteristic formula 1, variations in the drive transistor threshold voltage Vth lead to variations in the drain current Ids even if the gate voltage Vgs remains constant. This leads to variations in brightness from one pixel to another, thus degrading the screen uniformity. As a result, pixel circuits have been hitherto developed which incorporate the capability to cancel variations in the threshold voltage of the drive transistor T1. An example thereof is disclosed in Patent Document 2.
A pixel circuit incorporating the capability to cancel variations in the threshold voltage of the drive transistor T1 is capable of improving the brightness change caused by the change over time in the screen uniformity and the threshold voltage. However, as far as the characteristics of the TFT making up the drive transistor are concerned, not only the threshold voltage Vth but also the mobility μ are known to vary from pixel to pixel. Pixel circuits are known which incorporate the capability to correct the mobility p as well as the threshold voltage Vth. An example thereof is disclosed in Patent Document 3.